Switch design has received a significant attention and a large variety of switch architecture alternatives has been developed over several decades. Most structures reported in the literature, or used in practice, fall under one of two categories. The first is the multi-stage family of switches and the second is the time-multiplexed space-switch-based family of switches.
Several switch elements may be interconnected to create a modular switch having a capacity that is higher than the capacity of any of the constituent switch elements. A switch element may qualify as a building unit of a modular switch if it is internally contention free. A common-memory switch is an example of a contention-free switch. The ratio of the access capacity of the modular switch to the access capacity of the largest constituent switch element may be called the capacity gain. By necessity, the capacity of each constituent switch element may be considered to be divided into an access capacity and an inner capacity, where the access capacity (also called throughput) is the capacity available to users of the modular switch and the inner capacity is the capacity used for interconnection to other switch elements. A measure of the efficiency of a modular switch may be derived as the ratio of the aggregate access capacity of all constituent switch elements to the total capacity of all constituent switch elements. By these definitions, the efficiency of a single switch element is 1.0 and the capacity gain, G, of a single switch element is 1.0.
One of the popular modular structures is the multi-stage structure known as the Clos network, which comprises an odd number of stages (3, 5, 7, etc.), where a path from any ingress port to any egress port traverses a switch element in each stage. The ultimate capacity of a multi-stage Clos structure is determined by the number of stages and the sizes of its switch elements. The multi-stage Clos-type structure is usually limited to three stages. To reduce or eliminate internal blocking in a three-stage Clos switch, the inner side of each of the first and third switch elements is required to have a higher capacity than the corresponding outer side. With k denoting the number of ingress ports of a first-stage switch element or the number of egress ports of a third-stage switch element, switch elements of dimension N×N each would be used in the middle stage, where N is selected to be larger than k in order to provide an internal expansion to reduce internal blocking caused by misalignment of free channels; all ports in all switch elements are considered to be of equal capacity, e.g., 10 Gigabits per second (Gb/s) each. Switch elements of dimension k×N each would be used in the first stage, and switch elements of dimension N×k each would be used in the third stage. The dimension of the three-stage Clos switch is then (k×N)×(k×N), its capacity is k×N×R, where R is the rated capacity, in bits per second, of each ingress port or egress port. The capacity gain equals k and the efficiency E equals k/(k+2×N). In a data Clos switch, each of the switch elements may have a common-memory structure.
In order to realize a multi-stage switch of large dimension, switch elements of a relatively large dimension would be required. For example, to realize a switch of 8192×8192 using a three-stage structure, non-blocking switch elements of dimension 128×128 would be required.
A high-capacity switch that may use switch elements of relatively smaller dimensions can be realized using a space switch to interconnect the switch elements. A conventional time-multiplexed space switch using input buffers, and usually output buffers, may provide high scalability. Each input buffer may be paired with an output buffer and each paired input and output buffers may be included in an input/output module. The scalability of a conventional time-multiplexed space switch is determined by two factors. The first factor, and the more severe of the two, is the scheduling effort, which is traditionally based on arbitration among input ports vying for the same output port. The second factor is the quadratic fabric complexity of the space switch where structural complexity increases with the square of the number of ports. The capacity gain is determined by the dimension of the space switch and the dimension of an input/output module. Input/output modules each having multiple ports may connect to multiple space switches operating in parallel.
The capability and efficiency of a switching network are determined primarily by its switches and, because of this pivotal role of the switches, switch design continues to attract significant attention. It is desirable to construct modular large-scale switches using switch modules of a relatively small dimension in order to suit a variety of deployment conditions. Modular switches that scale from a moderate capacity, of 160 gigabits per second (Gb/s) having a dimension of 16×16 with 10 Gb/s input or output channels, to a high capacity of hundreds of terabits per second (Tb/s) having a dimension exceeding 16384×16384, using non-blocking switch elements each of a relatively small dimension (not exceeding 8×8, for example) would significantly facilitate the construction of efficient high-capacity networks of global coverage.